This invention relates to carbon nanotube materials and processes for their preparation. In particular the invention relates to substrate supported aligned carbon nanotube films, processes for their preparation and their use in the preparation of multilayer carbon nanotube materials. The invention also relates to the construction of devices from such materials for practical applications in many areas including as electron field emitters, artificial actuators, chemical sensors, gas storages, molecular-filtration membranes, energy-absorbing materials, molecular transistors and other optoelectronic devices.
Carbon nanotubes usually have a diameter in the order of tens of angstroms and the length of up to several micrometers. These elongated nanotubes consist of carbon hexagons arranged in a concentric manner with both ends of the tubes normally capped by pentagon-containing, fullerene-like structures. They can behave as a semiconductor or metal depending on their diameter and helicity of the arrangement of graphitic rings in the walls, and dissimilar carbon nanotubes may be joined together allowing the formation of molecular wires with interesting electrical, magnetic, nonlinear optical, thermal and mechanical properties. These unusual properties have led to diverse potential applications for carbon nanotubes in material science and nanotechnology. Indeed, carbon nanotubes have been proposed as new materials for electron field emitters in panel displays, single-molecular transistors, scanning probe microscope tips, gas and electrochemical energy storages, catalyst and proteins/DNA supports, molecular-filtration membranes, and energy-absorbing materials (see, for example: M. Dresselhaus, et al., Phys. World, January, 33, 1998; P. M. Ajayan, and T. W. Ebbesen, Rep. Prog. Phys., 60, 1027, 1997; R. Dagani, C&E News, Jan. 11, 31, 1999).
For most of the above applications, it is highly desirable to prepare aligned carbon nanotubes so that the properties of individual nanotubes can be easily assessed and they can be incorporated effectively into devices. Carbon nanotubes synthesised by most of the common techniques, such as arc discharge (Iijima, S. Nature 354, 56-68, 1991; Ebbesen, T. W. & Ajayan, P. M. Nature 358, 220-222, 1992) and catalytic pyrolysis, (See, for example: M Endo et al. J. Phys. Chem. Solids 54, 1841-1848, 1994; Ivanov, V. et al. Chem. Phys. Let. 223, 329-335, 1994) often exist in a randomly entangled state. However, aligned carbon nanotubes have recently been prepared either by post-synthesis manipulation (see, for example: Aegean, P. M., et al. Science 265, 1212-1214, 1994; De Heer, W. A. et Al. Science 268, 845-847) or by synthesis-induced alignment (see, for example: W. Z. Li, Science, 274, 1701, 1996; Che, G., Nature, 393, 346, 1998; Z. G. Ren, et al., Science, 282, 1105, 1998; C. N., Rao, et al., J.C.S., Chem. Commun., 1525, 1998).
Where the aligned nanotubes are prepared by post-synthesis manipulation, they are generally aligned in the plane parallel to the surface, whereas when the aligned carbon nanotubes are prepared by approaches which involve synthesis-induced alignment, the carbon nanotubes are aligned perpendicularly to the substrate. The choice of substrates upon which aligned carbon nanotubes may be grown is strongly limited by the conditions under which the nanotubes are synthesised. For many potential applications of aligned carbon nanotubes it would be convenient to provide the aligned carbon nanotubes on a substrate which is different from the substrate upon which the perpendicularly aligned carbon nanotubes are grown.
Further, multilayer structures built up from aligned carbon nanotubes are of vital interest, as the use of multilayered semiconductor materials and devices is highly desirable for many applications. Examples include the use of molecular-beam epitaxy for making superlattices consisting of the alternating layers of gallium arsenide and aluminium arsenide as hetero-structured semiconductor materials (M. A. Herman and H. Sitter, “Beam Epitaxy: Fundamentals and Current Status”, Springer-Verlag, Berlin, 1989), the use of Langmuir-Blodgett and chemical vapor deposition techniques for construction of organic field-emission transistors (M. F. Rubner and T. A. Skotheim, in “Conjugated Polymers”, J. L. Brédas and R. Silbey (eds.), Kluwer Academic Publishers, Dordrecht, 1991; G. Horowitz, Adv. Mater., 10, 365, 1998), and the use of layer-by-layer adsorption and solution-spinning methods for preparing multilayer thin films of conjugated polymers as organic light-emitting diodes (S. A. Jenekhe and K. J. Wynne, “Photonic and Optoelectronic Polymers”, ACS Sym. Ser. 672, ACS Washington, D.C., 1995; L. Dai, J. Macromole. Sci., Rev. Macromole. Chem. Phys. 1999, 39(2), 273-387). The overall properties of multilayer materials and/or devices are highly dependent not only on the intrinsic properties of the constituent materials in each of the layers but also the particular layer stacking sequence, and the interface and surface structures, thus adding additional parameters for the design and control of their behaviours. Accordingly there is a need for a method of transferring aligned carbon nanotube films from the substrate on which they are synthesised to other substrates.
It has now been unexpectedly found that aligned carbon nanotubes can be readily peeled off the substrate on which they are synthesised by applying a layer of a second substrate to the top surface of the aligned carbon nanotube layer and peeling off the aligned carbon nanotubes together with the second substrate.